1. Field of the Invention
This application relates to systems and methods for measurement of a variety of parameters, e.g., strain, temperature, pressure, etc. More particularly, it concerns such systems and methods that comprise the combination of both polarimetric and a few-mode sensor in a single optical fiber to provide a large dynamic range in measuring a parameter without sacrificing sensitivty.
2. Description of the Prior Art
The use of optical fibers for the sensing of strain is well-known and this has been done in a variety of applications, e.g., see U.S. Pat. Nos. 4,295,738; 4,611,378; 4,653,906 and 4,947,693.
Several different classes of optical fiber sensors have been investigated, each having particular advantages and disadvantages. Two classes of particular interest are the polarimetric sensors and the so-called "few-mode" or bimodal sensors. In the polarimetric sensors, a birefringent fiber, i.e., an Andrew E-type fiber, is used to conduct two orthogonally polarized optical signals longitudinally along the fiber, each with a different propagation velocity. As the fiber is strained, a change in the relative optical path length results, thereby rotating the resultant polarization at the fiber output. The polarization rotation is then converted to an intensity variant signal using a linear polarizer at the output end. In a typical response with a polarimetric sensor, where the output intensity signal is plotted versus the induced strain nearly 2000 microstrain is required to observe a complete transition from minimum to maximum detected intensity.
The "few-mode" or bimodal sensor is unlike the polarimetric sensor in that polarization effects are not required for sensing strain. In this case, an optical fiber is used that will support two or more lower order propagation spatial modes. As the fiber is subjected to longitudinal strain, the effective propagation constant for each of the modes is altered in such a way that the relative phase between each mode is shifted in proportion to the strain. Thus, at the output end of the fiber, both modes interfere producing an intensity pattern in space which when detected at a particular point, varies in proportion to the induced strain. In a typical response, a 150 microstrain is sufficient to cause a complete transition from minimum to maximum detected intensity. As the applied strain is increased, the intensity pattern alternates through successive light/dark transitions.